This invention relates to reel-to-reel magnetic tape systems. More particularly, the invention relates to a two mode control system for controlling the amount of tape in the buffers of a reel-to-reel tape system that uses a minimum amount of power, does not cause the tape to overshoot its final position, and that can handle a worst case capstan reversal with a minimal turnaround delay.
A reel-to-reel magnetic tape system has two reels, a file reel and a machine reel. The file reel has a continuous length of tape wound on it from which the magnetic tape system may read data or on which the magnetic tape system may write data. The tape on the file reel is threaded through a path that carries the tape past a set of read/write heads to the machine reel. The machine reel stores the tape from the file reel after it has passed the read/write heads. Tape may be rewound from the machine reel back onto the file reel.
A capstan roller, driven by a capstan motor (the combination of capstan roller and capstan motor will hereafter be called the capstan), is typically used to drive the magnetic tape past the read/write heads. As the capstan moves the tape past the read/write heads, the file and machine reels must turn, one to supply and the other to store it.
A capstan accelerates to its final velocity in a time that is very short compared to the time it takes the reel motors to start moving the reels. Therefore, buffers are provided in the tape path that supply the tape on one side of the capstan and store it on the other side until the reel motors can get their respective reels up to speed.
There are two types of buffer systems commonly used on reel-to-reel magnetic tape systems: (1) vacuum columns, and (2) tension arms (also called swing arms). Both types use servo systems to control the position of the tape in the buffers. The theory and operation of tape buffers is well understood in the art and will not be discussed herein.
In the prior art, two types of servo systems are commonly used to control the movement of tape from one reel to the other, including controlling the position of the tape in the tape buffer. In a first, a linear position controller provies a correction current to the reel motor that is proportional to the position error of the tape within the tape buffer and to the velocity of the tape. In a second, a "bang-bang" controller provides a current pulse to the reel motor in a drive-coast-brake sequence, a current pulse of one polarity serving to drive the motor in one direction, and a current pulse of the opposite polarity serving to brake or stop the motor once driven.
Unfortunately, both the linear position and "bang-bang" servo controllers have disadvantages. The linear position controller cannot respond to the worst case capstan reversal wherein the capsan, while driving the tape in one direction, is reversed without a pause. When this happens, the linear position controller must hold off the reversal, i.e., introduce a pause, while the servo adjusts the amount of tape in the buffers. This results in a degradation in system performance.
The "bang-bang" controller is wasteful of power since it is continually accelerating and decelerating the reel motor. The "bang-bang" controller may or may not be able to handle the worst case reversal without causing a degradation in performance, depending upon the tape speed at the time the reversal occurs; but in those cases where a reversal can be handled, the tape will often overshoot its final position. This also wastes power as the overshoot must be brought under control.